The preparation of cyclic esters of alpha-hydroxycarboxylic acids is an old and much studied process. Heretofore, the preparation has been conducted in two distinct batch steps involving first preparing an oligomer of the hydroxycarboxylic acid, i.e., a relatively short-chain condensation polymer thereof, then heating the polymer under reduced pressure to generate the desired cyclic ester. Gruter et al., U.S. Pat. No. 1,095,205 (1914); Lowe, U.S. Pat. No. 2,668,162 (1954); Bhatia, U.S. Pat. No. 4,835,293 (1989); Bellis, U.S. Pat. No. 4,727,163 (1988); Muller, Ger. Pat. Applications 3632103 and 3708915 (1988). Such processes spanning over 70 years of technology suffer in that they require hours of reaction time at high temperatures for the preparation of the polymeric intermediate and its thermolysis to the cyclic ester. Further, the rather long residence times at the high temperatures employed often results in side reactions leading, for example, to unwanted isomers, charring of the polymer and consequently difficult to handle reactor heels.
It is an object of this invention to provide a novel essentially single-step continuous process for the rapid conversion with high productivity of an alpha-hydroxycarboxylic acid or ester thereof to a cyclic ester in a single reaction zone without need to separately prepare an oligomer intermediate to the cyclic ester.
It is another object to provide such a process that is conducted with the carboxylic acid material being fed in the vapor phase to a reaction zone containing a solid catalyst for the conversion to the dimeric cyclic ester.
Still another object is to provide a process as above that is gas-assisted, the gas serving to facilitate the feeding of the carboxylic material to the reaction zone as well as the removal of the resultant cyclic ester therefrom.
Yet another object is to provide such continuous catalyzed vapor phase process for the production of glycolide and lactide, in particular L-lactide from L-lactic acid or an alkyl lactate.